The Student Room Group
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>

Wavelength

Like how am i supposed to know if a certain colour has a greater wavelength than another colour, like wavelength of red > wavelength of white?
Original post by Ayaz789
Like how am i supposed to know if a certain colour has a greater wavelength than another colour, like wavelength of red > wavelength of white?


First of all, white light doesn't have a wavelength, it's a combination of all the wavelengths of visible light. Secondly, you just have to learn it. Memorise this diagram:
Reply 2
Avatar for Lychee628
Lychee628
OP
19
Original post by Plagioclase
First of all, white light doesn't have a wavelength, it's a combination of all the wavelengths of visible light. Secondly, you just have to learn it. Memorise this diagram:

Okay ill learn that diagram thanks!
So therefore according to that the wavelength of blue> wavelength of red
& the wavelength of any colour> wavelength of white?
Original post by Ayaz789
Okay ill learn that diagram thanks!
So therefore according to that the wavelength of blue> wavelength of red
& the wavelength of any colour> wavelength of white?


No the wavelength of red (~700nm) is longer than the wavelength of blue (~450nm). White light doesn't have a wavelength, the whole point is that it's a combination of different wavelengths of light. It makes no sense to say that something has a longer wavelength than white light because white light doesn't have a wavelength, it's a continuum of wavelengths.
Original post by Ayaz789
Okay ill learn that diagram thanks!
So therefore according to that the wavelength of blue> wavelength of red
& the wavelength of any colour> wavelength of white?


Small wavelengths are at the top of the diagram. An angstrom is smaller than a kilometre.
Reply 5
Avatar for Lychee628
Lychee628
OP
19
Original post by Plagioclase
No the wavelength of red (~700nm) is longer than the wavelength of blue (~450nm). White light doesn't have a wavelength, the whole point is that it's a combination of different wavelengths of light. It makes no sense to say that something has a longer wavelength than white light because white light doesn't have a wavelength, it's a continuum of wavelengths.


http://www.egsphysics.co.uk/files/a_level_past_papers_new/Jan%2009/AQA-PHYA2-W-MS-JAN09.PDF
So why in this ms does it say that in q7b?
Original post by Ayaz789


That's the average wavelength of the white light. If the mark scheme was talking about the wavelength of the white light then it wouldn't make sense because there are infinitely many different wavelengths for the white light.
White light has a continuous distribution of wavelengths, but you can still take the average of them(in every case as far as I know). If you've studied calculus then if the allowed values of λ\lambda in the white light are in I then λˉ=∫IλP(λ)dλ∫IP(λ)dλ\bar{ \lambda} = \dfrac{\displaystyle\int_I \lambda P(\lambda)\mathrm{d}\lambda}{ \displaystyle\int_I P(\lambda)\mathrm{d}\lambda}. If not then imagine taking the mean of a finite number of (weighted) values then extending this to infinitely many.
(edited 7 years ago)
Reply 7
Avatar for Lychee628
Lychee628
OP
19
Original post by morgan8002
That's the average wavelength of the white light. If the mark scheme was talking about the wavelength of the white light then it wouldn't make sense because there are infinitely many different wavelengths for the white light.
White light has a continuous distribution of wavelengths, but you can still take the average of them(in every case as far as I know). If you've studied calculus then if the allowed values of λ\lambda in the white light are in I then λˉ=∫IλP(λ)dλ∫IP(λ)dλ\bar{ \lambda} = \dfrac{\displaystyle\int_I \lambda P(\lambda)\mathrm{d}\lambda}{ \displaystyle\int_I P(\lambda)\mathrm{d}\lambda}. If not then imagine taking the mean of a finite number of (weighted) values then extending this to infinitely many.


I have studied calculus but that does look difficult haha :smile: Okay so what is the wavelength of average white?:redface:
Original post by Ayaz789


It says average... red is at one of the extreme ends of the visible spectrum the middle of a fringe made with white light would be in a different place to the middle of a fringe made with red light.

fwiw A couple of mnemonics for the order of colours by decreasing wavelength

Richard Of York Gave Battle In Vain
Rock On You Great Big Indian Vole
Reply 9
Avatar for Lychee628
Lychee628
OP
19
Original post by Joinedup
It says average... red is at one of the extreme ends of the visible spectrum the middle of a fringe made with white light would be in a different place to the middle of a fringe made with red light.

fwiw A couple of mnemonics for the order of colours by decreasing wavelength

Richard Of York Gave Battle In Vain
Rock On You Great Big Indian Vole

Okay thanks ill remember them!
Reply 10
Avatar for Lychee628
Lychee628
OP
19
Original post by Joinedup
It says average... red is at one of the extreme ends of the visible spectrum the middle of a fringe made with white light would be in a different place to the middle of a fringe made with red light.

fwiw A couple of mnemonics for the order of colours by decreasing wavelength

Richard Of York Gave Battle In Vain
Rock On You Great Big Indian Vole

By decreasing wavelength you mean wavelength of red is greater than wavelength of blue?
Original post by Ayaz789
I have studied calculus but that does look difficult haha :smile: Okay so what is the wavelength of average white?:redface:


It depends on the specific distribution of the light and is given by the equation I wrote above. It'll be somewhere below red and above blue.
Reply 12
Avatar for Lychee628
Lychee628
OP
19
Original post by morgan8002
It depends on the specific distribution of the light and is given by the equation I wrote above. It'll be somewhere below red and above blue.


Okay np
Original post by Ayaz789
By decreasing wavelength you mean wavelength of red is greater than wavelength of blue?


yes



Original post by Ayaz789
Okay so what is the wavelength of average white?:redface:


I'll stick my neck out and say it's about 540 or 550 nm for practical purposes... which is about the colour of a green LED
Reply 14
Avatar for Lychee628
Lychee628
OP
19
Original post by Joinedup
yes





I'll stick my neck out and say it's about 540 or 550 nm for practical purposes... which is about the colour of a green LED

Thanks haha

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you